'The Golden Age of Heavy Ion Collisions'

Nuclear physicists conducting research at the Relativistic Heavy Ion Collider (RHIC) traded shift time for presentations on the latest successes and plans for the future at the 2019 RHIC & AGS (Alternating Gradient Synchrotron) Users’ Meeting June 4-7. Even as RHIC’s beams continued to collide for the completion of Run 19, there was plenty to celebrate in terms of machine performance and scientific highlights at this U.S. Department of Energy Office of Science user facility for nuclear physics research at Brookhaven National Laboratory.

“RHIC continues to be an incredible machine, performing better than ever before and the most capable facility for this work,” said Berndt Muller, Brookhaven Lab’s Associate Director for Nuclear and Particle Physics, at a plenary session summarizing the most recent accomplishments on June 6.

According to Brookhaven Lab Director Doon Gibbs, who welcomed attendees that day, carrying out the RHIC research program and participating in the national effort for developing a possible U.S.-based Electron-Ion Collider (EIC) is the highest priority for the Lab. The RHIC program, like the other major research areas Gibbs noted, dovetails with the Laboratory’s vision of delivering “discovery science and transformative technology that power and secure our nation’s future.”

“These meetings are important for a number of reasons: One is it gives [us] a chance to tell you what’s going on at RHIC, but also what’s new at the Laboratory, what we’re thinking about, and to get your feedback. On the other hand it gives a chance for us to hear from you—about your results, what’s new, what’s exciting. We like to hear about what is working at Brookhaven, but we also need to hear and want to hear about what isn’t. So thank you very much for coming,” he said.

Gibbs was optimistic about the future at the Lab. He highlighted progress on the Science User and Support Center (SUSC) building—the first building in the Lab’s proposed Discovery Park complex—and plans to relocate a Long Island Railroad station closer to the site as amenities that will enhance users’ experiences. And he noted that despite a change in vendor, food service will continue at the Lab.

Gibbs capped his talk by showing the Lab’s new promotional video, which heavily features RHIC, saying, “It’s an exciting time to be associated with the national labs.”

Meeting highlights

Mueller presented a summary of “all the fantastic work you have all been doing in the past and what you are gearing up to do.”

This included a series of results from RHIC’s PHENIX collaboration on the formation of tiny specks of quark-gluon plasma—a soup of matter’s most fundamental building blocks—in collisions using a series of small projectiles crashing into larger gold nuclei. There were also results from both the PHENIX and STAR detectors analyzing collisions of polarized protons. These “spin physics” experiments are designed to investigate how the quark, antiquark, and gluon components of a proton each contribute to that particle’s intrinsic spin. Mueller also gave an overview of STAR’s goal in the current run and over the next three years to “explore in more detail than ever before the phase diagram of nuclear matter.” This program, known as the Beam Energy Scan II (BES-II), is searching for details of how quarks and gluons transition to more ordinary forms of nuclear matter in collisions of gold ions at a range of low energies.

In presenting these findings, Mueller repeatedly noted that “these data can only be taken at RHIC”—a testament to the collider’s unmatched versatility for exploring the most intriguing questions in nuclear physics.

He described some of the recent outstanding machine performance achievements, including higher collision rates than planned, and a first-in-the-world accomplishment of “bunched-beam electron cooling” that will be essential to next year’s continuation of BES-II.

In later talks describing various techniques used to maximize collision rates—including commissioning tests for the Low Energy RHIC electron Cooling (LEReC) accelerator system that will be used for bunched beam cooling next year—various representatives of the Collider-Accelerator Department (C-AD) and STAR all expressed their appreciation for the cooperative spirit that allowed both groups to achieve their goals during the run.

RHIC’s detectors have also evolved and continue to undergo improvements to make use of ever-increasing collision rates and track particles emerging from collisions as never before.

As Mueller noted, “Detector upgrades are in place for STAR and performing fantastically well.” These include brand new inner sectors for STAR’s Time Projection Chamber, and improvements to the Event Plane Detector and Endcap Time of Flight detector, which allow physicists to track particles emerging at angles closer to the beamline and with low momentum that might otherwise be lost.

“C-AD worked hard and provided us with very stable beams,” said STAR collaborator Irakli Chakaberia of Kent State University. “They minimized the losses and maximized the up time for STAR’s data taking. Good communication between C-AD and STAR plays a huge role in delivering the beautiful conditions.”

Additional upgrades for tracking particles at extreme forward angles may be on the way by 2022, Mueller noted, suggesting that there had been “encouraging news” from the National Science Foundation (NSF) as a possible source of funding.

Meanwhile, PHENIX, RHIC’s other major detector, is being transformed into a radically upgraded detector called sPHENIX. “This is an exciting part of the future,” Mueller said. “The collaboration is expanding and going full-blast.” The team building the sPHENIX detector just completed a very successful review, which enables the start of construction with DOE funding for this state-of-the-art instrumentation capable of capturing data at unprecedented rates.

Both STAR and sPHENIX will also have excellent capabilities for exploring “cold” nuclear matter—the properties of the nucleus absent the creation of superheated quark-gluon plasma. This includes the search for more details about the proton spin puzzle in future polarized proton runs. Results presented in a summary session on June 7 by Renee Fatemi of the University of Kentucky indicate an important though still uncertain contribution from gluons and intriguing “asymmetric” contributions of antiquarks that appear to depend on their “flavors”—antiup vs. antidown.

Many of the speakers emphasized the important contributions from many collaborating institutions across the nation and around the world. As Mueller said, this as “truly a distributive effort and a strength.”

“We have a fantastic program going through the mid 2020s,” he said.

A future Electron-Ion Collider?

Mueller also spoke about the proposed Electron-Ion Collider (EIC). The EIC would be a particle accelerator that collides electrons with protons and heavier nuclei, which will make manifest the precise arrangements of the constituent quarks and gluons that make up the protons and the nuclei—an important step in nuclear physics.

A recent National Academy of Sciences report endorsed the “compelling scientific case,” and highlighted advances in accelerator science and technology as well as benefits to other fields from medicine to materials science that would flow from developing the project, Mueller said.

A location for the EIC in the U.S. has not yet been determined. But independent of where an EIC might be built, “this is the time to get involved in the user community and the design of the detectors,” Mueller said.

Stony Brook University’s Abhay Deshpande, who also serves as director of Electron-Ion Collider science at Brookhaven, gave a much more detailed presentation of EIC science and potential machine capabilities in a separate plenary talk on June 7.

“We are pushing the parameters of detector acceptance and machine performance beyond what has been done,” Deshpande said, noting that “any time [physicists have gone] beyond the limits, we have discovered new things. This machine will push the limits.”

Reports from the agencies

Additional information on the status and outlook for the field came in presentations from the DOE and NSF, the agencies in Washington, D.C., that fund RHIC operations and the bulk of its research program.

Both Timothy Hallman, the Associate Director for Nuclear Physics (NP) in the DOE Office of Science, and Richard Witt, the Program Manager for Heavy Ion Physics in NP, extolled the value of the research taking place in nuclear physics at Brookhaven Lab and across the DOE complex.

“One of my favorite things about the job is to get out in the field and speak directly with the people doing the great work that we talk about all the time,” Hallman said.

Each DOE presentation highlighted some of RHIC’s successes, and also the importance of the DOE Isotope Program for increasing the U.S. inventory of enriched stable isotopes. Many of these isotopes are used for medical diagnostics and treatments, and some are produced at Brookhaven’s Linac Isotope Producer, which operates using the same accelerator infrastructure as the RHIC physics program.

Hallman and Witt noted progress on nuclear physics goals laid out in the field’s most recent Long Range Plan, in 2015.

“The successes are entirely due to you” in achieving output from RHIC and other existing NP investments, Witt said.

Of particular note: The Facility for Rare Isotope Beams (FRIB) at Michigan State University is nearing completion, and an Electron-Ion Collider was identified as the next priority facility for the field by the Nuclear Science Advisory Committee. Hallman explained that there is a line in the President’s fiscal year 2020 budget request that can support things like EIC pre-conceptual design efforts and R&D.

“That’s a statement by the U.S. federal government that they are serious about the possibility of an Electron-Ion Collider and they’re willing to spend money on ‘other project costs’ for it. And the Office of Science is very serious about the possibility of constructing this facility,” Hallman said. But the ultimate decision of whether or not to move forward—a declaration of “mission need” or Critical Decision 0—rests with the Deputy Secretary of Energy.

Beyond the EIC, the overall budget picture for the next fiscal year is far from resolved. After reviewing the presidential request, which is down about $65 million for NP compared to what was appropriated last year, both the U.S. House of Representatives and the Senate will have their say in deciding what is ultimately allotted to the various federal agencies.

“We have to maintain balance between research and facility operations. And we have to deliver compelling science at any level of funding,” Hallman emphasized.

“What got us here is the great research you folks do and the benefits it’s had for society and the world. The U.S. still has unquestioned leadership in QCD [quantum chromodynamics, the theory describing the interactions of quarks and gluons in nuclear matter]. Articulating the value of what you do is just as important as it ever was.

“Delivering exciting discoveries, important scientific knowledge, technological advances, and workforce training is what we do best, and we need to keep up the good work.”

Jim Thomas, the NSF program director for Nuclear Physics, echoed that sentiment, noting that in his case, only 15-20 percent of proposals across the agency get funded. But the accomplishments he presented, including the detection of gravitational waves and the first photographic evidence of a black hole, can capture the public’s imagination and have enormous impact.

“Things that were fiction yesterday are true science today,” he said.

The keys to a successful proposal with NSF, he said, are extraordinary science and compelling broader impact. Those broader impacts can include education and outreach that help build the workforce, broaden participation in science, or that advance science in other fields or the economy.

There are a range of funding opportunities for nuclear physics, including for Major Research Instrumentation at various funding levels, with details on the NSF website. Thomas encourages RHIC groups to apply.

“The availability of funds in the National Science Foundation depends on the number of proposals submitted,” he said. So, more physics proposals bring more money into the physics division, which increases that division’s competitiveness across the whole of NSF.

Noting the uncertainty of the budget process, he advised everyone to “Do good science, acknowledge agencies [that fund you], and pursue broader impacts of your work. When possible, please, bring the community into your laboratory and give them a tour. And talk to the 10-year-old next door!”

Diversity, career advice, and tributes

Over the course of the four-day meeting, many additional speakers presented details on the physics topics highlighted above. Slides to many of the presentations can be found on the Users’ Meeting website.

In addition, the entire afternoon of June 5 was devoted to talks on diversity and career development. These included presentations aimed at giving recognition to the value of inclusion in bringing new perspectives to physics, personal experiences, and information about resources for women and members of the LGBTQ+ community. One session included advice on navigating and improving the world of physics and its research environment, while another presented editing at a physics journal as an avenue for applying physics expertise.

The coffee break that afternoon was dedicated to remembering Christina Swinson-Cruz, deputy head of Brookhaven Lab’s Accelerator Test Facility (ATF)—another of the Lab’s Office of Science user facilities where scientists have tested innovative accelerator technologies for use at RHIC and around the world. Swinson-Cruz, who died last December at the age of 39, was a reliable presence at RHIC & AGS Users’ Meetings and a champion for women in science throughout her career.

Another tribute took place as part of Thursday’s plenary session, when Columbia University professor and RHIC collaborator William Zajc offered homage to renowned theoretical physicist Roy Glauber, who died last year at the age of 93, and was one of the leaders Zajc sought out for advice when he was first entering the field.

“Glauber modeling plays a central role in the type of physics that we do,” said Zajc, laying out the connections between Glauber’s foundational publications of the 1960s and the study of heavy ion interactions—and how citations for those papers rose dramatically after RHIC operations began in 2000.

“Light played a role in everything he did,” Zajc said – from photography, to microscopes, to telescopes, to the development of the quantum theory of optical coherence that garnered him the 2005 Nobel Prize.

Glauber’s interest in using probes of wavelength smaller than the ranges of interactions proved to be the essential tool for modeling the initial stage of nuclear collisions at RHIC, and his work on the quantum mechanics of multi-photon processes led to essential techniques used to understand the final stage in the evolution of quark-gluon plasma.

“Roy was a friend to the heavy-ion community,” Zajc said. “Our sorrow is tempered by knowing that [his] light is now shining from a higher place.”

Awards and Elections

One of the highlights of the annual meeting is the recognition of outstanding theses and poster presentations by students who will likely be future leaders in the field, and the election of new officers to serve the community on the Users Executive Committee.

Berndt Mueller announced thesis awards for the following two winners:

Jonah Bernhard of Duke University for a thesis titled: “Bayesian parameter estimation for relativistic heavy-ion collisions”

“It is hard to overestimate the importance of these new theory developments in connecting theory to data,” Mueller said.

Javier Orjuela-Koop of the University of Colorado at Boulder for a thesis titled: “On kinetic transport in small system collectivity and a measurement of separated open heavy flavor production in p+p collisions at √sNN = 200 GeV”

“This thesis had two parts, each of which would have made a complete thesis,” Mueller noted. “It’s a model thesis—and an ideal that future generations of students can strive for.”

Mueller also announced honorable mentions for two additional outstanding thesis projects he called “important and impactful.” The first, by James Daniel Brandenburg of Rice University, involved “an enormous amount of careful data analysis,” Mueller said. The second, by Isaac Upsal of Ohio State University, required “novel and difficult data analysis related to the vorticity of the quark gluon plasma.” As an added benefit to the students and the Lab, both honorees are now working as postdocs at Brookhaven, supported jointly by the Lab and Shandong University for their work on the STAR experiment.

Vladislav Zakharov, a student at Stony Brook University, earned the top honor in this year’s poster session.

This year continued the presentation of Merit Awards, given to scientists who earned their Ph.D. within the previous 10 years for significant contributions to the research at RHIC, the AGS, NASA Space Radiation Laboratory (NSRL), Tandem Van de Graaff, Brookhaven Linac Isotope Producer (BLIP), or the ATF. This year’s honorees are: Catherine Davis, Nicholas Dover, Alexander Jentsch, Zhongbo Kang, Wangmei Zha, Chi Yang, Shuai Yang, Jie Zhao, Joseph Osborn, and Timothy Rinn.

And flashing back to Tim Hallman’s talk, there was one more award worth noting: recognition of accelerator physicist Alexei Fedotov for his outstanding project management of the Low Energy RHIC electron Cooling (LEReC) accelerator improvement project.

“You can have all the resources you need, but unless you have really good, talented people who get things done, you’re just not going to be successful,” Hallman said. “Your leadership was essential to successfully fabricate and install this instrument and will have a significant impact on the nuclear physics program at BNL and the national heavy-ion physics program. The Office of Science appreciates your hard work and dedication,” he said, reading from the citation signed by J. Steven Binkley, acting director of the Office of Science at the time of the award.

At the conclusion of the meeting, RHIC & AGS Users’ Executive Committee (UEC) Chair Rosi Reed of Lehigh University announced the UEC election results. The UEC represents scientists whose research makes use of five user facilities at Brookhaven: RHIC, the AGS, Tandem, ATF, and NSRL. The group provides an organized channel for discussions between these scientists and the Lab's administration. The UEC is also primarily responsible for organizing the annual users' meeting, with support from Brookhaven's Guest, User & Visitor Center and others across the Lab site.

Ron Belmont, University of North Carolina Greensboro, was elected incoming chair.

Brookhaven National Laboratory is supported by the U.S. Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.

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